The present invention relates to methods and apparatus for bonding of filamentary type wires and more particularly concerns such methods and apparatus for making improved multiple bonds of a single wire to a semi-conductor device.
In the manufacture of semi-conductor devices, small diameter wires, often having a diameter considerably less than the diameter of a human hair, are attached ultrasonically or by thermal compression to and between various contact areas on the devices. These filamentary wires and their connections are a major cause of reliability problems encountered in such semi-conductive devices. Problems include poor metallization, variable wire cross-section, improperly mounted chips, and machine variables including holding fixtures, among others. All of these difficulties may contribute to a faulty bond of the connecting wire.
Inadequacies of the bond produced by prior art ultrasonic techniques are fundamentally two in number. The first of these is termed "heel cracking" or "necking". This is caused by an increased deformation, e.g. decreased cross-sectional area and concomitantly increased stress concentration, at the inner end of the first bond of a wire that is to be bonded between two points. Consequently, upon subsequent handling of the wire or the bonded device, the wire may break at the point of increased stress concentration, causing an undesirable open circuit.
A second bonding problem is associated with heavy wire cut-off. The term "heavy wire", as employed with regard to semi-conductor connecting wires, refers to wires in the order of five to twenty thousandths of an inch in diameter, whereas the term "light wire" refers to wires generally less than five thousandths of an inch, and normally not greater than two thousandths of an inch in diameter.
Upon completion of the second bond of a heavy wire, the wire must be parted, or cut, at the outer end of the second bond as close to the bond and contact pad as possible. This is done to avoid a protruding tail that may interfere with other closely adjacent contact pads or wires. In some arrangements, it is necessary to provide a separate cut-off knife and thus, an additional step is required for parting of heavy wire. Other prior arrangements for heavy wire parting are shown in the patents to Reber U.S. Pat. No. 3,347,442 and Machino et al. U.S. Pat. No. 3,648,354.
Typical apparatus for ultrasonic wire bonding in which the bonding wedge is rocked for the second bond cut-off are shown in the patents to Pennings U.S. Pat. Nos. 3,305,157 and 3,328,875.
The cut-off blade is complicated, expensive, difficult to repair and adjust, potentially destructive to the semi-conductive devices, and is easily damaged. In tilting machines, attempts have been made to decrease the effects of the necking or heel cracking by tilting the bonding wedge forward during the first bond and rearwardly during the second bond. However, because of practical mechanical considerations, the amount of tilt is so small as to be of questionable value.
In the patents to Eltzroth et al. U.S. Pat. Nos. 3,689,983 and 3,718,272, a bonding wedge is provided with forward and aft longitudinally extending relief grooves for the purposes of minimizing necking of the wire at the bonds. Although this arrangement may somewhat decrease the necking problem, it can only aggravate the wire cut-off problem, particularly with regard to heavy wire, because the very arrangement that is directed toward solution of the necking problem inherently intensifies the cut-off problem.
Recognizing the conflict between heel cracking or necking on the one hand and wire cut-off requirements on the other, U.S. Pat. No. 3,627,192 to Killingsworth suggests a balance between a sharp corner which would facilitate breaking the wire, and a rounded corner which would reduce imprinting the wire, but which makes parting more difficult. Such a solution is a compromise at best and does not optimumly solve either of the two problems.
Accordingly, it is an object of the present invention to bond a wire at a first point with improved wire strength and at a second point with improved parting characteristics.